diff --git a/lib/kolab_sync_timezone_converter.php b/lib/kolab_sync_timezone_converter.php
old mode 100644
new mode 100755
--- a/lib/kolab_sync_timezone_converter.php
+++ b/lib/kolab_sync_timezone_converter.php
@@ -393,6 +393,15 @@
 
         $timezoneInfo = unpack($timezoneUnpackString, base64_decode($_packedTimezoneInfo));
 
+        if ($timezoneInfo['standardHour'] == 23 && $timezoneInfo['standardMilliseconds'] == 999
+            && $timezoneInfo['standardMinute'] == 59 && $timezoneInfo['standardSecond'] == 59
+        ) {
+            $timezoneInfo['standardHour'] = 24;
+            $timezoneInfo['standardMinute'] = 0;
+            $timezoneInfo['standardSecond'] = 0;
+            $timezoneInfo['standardMilliseconds'] = 0;
+        }
+
         return $timezoneInfo;
     }
 
@@ -408,6 +417,16 @@
             return null;
         }
 
+        // According to e.g. https://docs.microsoft.com/en-us/windows/win32/api/minwinbase/ns-minwinbase-systemtime,
+        // 24 is not allowed in the Hour field, and consequently Outlook can't deal with it.
+        // This is the same workaround that Outlook applies.
+        if ($_timezoneInfo['standardHour'] == 24) {
+            $_timezoneInfo['standardHour'] = 23;
+            $_timezoneInfo['standardMinute'] = 59;
+            $_timezoneInfo['standardSecond'] = 59;
+            $_timezoneInfo['standardMilliseconds'] = 999;
+        }
+
         $packed = pack(
             "la64vvvvvvvvla64vvvvvvvvl",
             $_timezoneInfo['bias'],
diff --git a/tests/timezone_converter.php b/tests/timezone_converter.php
old mode 100644
new mode 100755
--- a/tests/timezone_converter.php
+++ b/tests/timezone_converter.php
@@ -103,6 +103,18 @@
         $this->assertSame($output['daylightMonth'], 3);
         $this->assertSame($output['daylightDay'], 5);
         $this->assertSame($output['daylightHour'], 0);
+
+        $output = $converter->getOffsetsForTimezone('Asia/Tehran', $datetime);
+
+        $this->assertSame($output['bias'], -210);
+        $this->assertSame($output['standardBias'], 0);
+        $this->assertSame($output['standardMonth'], 9);
+        $this->assertSame($output['standardDay'], 3);
+        $this->assertSame($output['standardHour'], 24);
+        $this->assertSame($output['daylightBias'], -60);
+        $this->assertSame($output['daylightMonth'], 3);
+        $this->assertSame($output['daylightDay'], 4);
+        $this->assertSame($output['daylightHour'], 0);
     }
 }